The Small-World Experiment

Consider this: You’re having coffee near the Trevi Fountain in Rome. You overhear two Americans chatting next to you and you ask where they’re from. One of them says she’s from Sioux City, Iowa. You say you’ve got a friend from Sioux City and it turns out to be your new acquaintance’s cousin. The culturally appropriate reaction at this point is for everyone to say, ‘‘Wow, what a small world!’’

Stanley Milgram (1967) invented an experiment to test how small the world really is. He asked a group of people in the midwestern United States to send a folder to a divinity student at Harvard University, but only if the participant knew the divinity student personally. Otherwise, Milgram asked people to send the folders to an acquaintance whom they thought had a chance of knowing the ‘‘target’’ at Harvard.

The folders got sent around from acquaintance to acquaintance until they wound up in the hands of someone who actually knew the target—at which point the folders were sent, as per the instructions in the game, to the target. The average number of links between all the ‘‘starters’’ and the target was about five. It really is a small world.

No one expects this experiment to actually happen in real life. It’s contrived as can be and it lacks control. But it’s compelling because it says something about how the natural world works. The finding was so compelling that it was the basis for the Broadway play ‘‘Six Degrees of Separation,’’ as well as the movie of the same name that followed and the game ‘‘Six Degrees of Kevin Bacon.’’ It also provoked research, first by social scientists and then by physicists and mathematicians, on the structure of relations in everything from people to corporations to nations to neurons to sites on the Internet. Looking for the structure of relations is another way of saying ‘‘network analysis.’’

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